1. Field of the Invention
The present invention generally relates to a tuning method and apparatus, and particularly relates to a tuning apparatus such as a TV tuner or the like provided in a TV receiver for channel-selecting a specific TV broadcast station transmitting a signal with a predetermined specific frequency.
2. Description of the Prior Art
Conventionally, a broadcasting service such as TV broadcasting has been based on a channel plan (a frequency assignment plan) whereby an available carrier frequency is assigned in advance for every broadcast station so as to utilize effectively a predetermined frequency band. Therefore, when the operator of a TV receiver desires to receive the broadcast of a specific broadcast station, he selects the desired specific broadcasting station out of a multiplicity of such stations, generally 12 broadcast stations, by operating a channel switch which has been preset to frequencies assigned to specific broadcast stations, for example, to center frequencies assigned to specific broadcast stations, in accordance with the channel frequency assignments.
In the case of broadcasting, such as TV broadcasting or the like, two broadcast stations having channels of the same center frequency sometimes exist in neighboring locations. In such a case, there is a possibility of radio interference between the two broadcast stations. Such interference phenomenon has been heretofore prevented by shifting the carrier frequencies of the two broadcast stations so as to make the difference between the carrier frequencies equal to an odd number of half the horizontal scanning frequency (hereinafter, simply referred to as "offset").
This results in a broadcast station which broadcasts on a carrier frequency different from the center frequency determined on the basis of the channel plan, that is, it broadcasts on a carrier frequency which is "offset" from the center frequency of the channel plan.
Recent TV receivers include a frequency synthesizing system in which a PLL (phase locked loop) is used to perform tuning by generating, in a digital manner, a signal having a center frequency predetermined for every reception channel in accordance with the channel plan. Such systems provide very stable reception. In the case where reception/selection is made for a broadcast station that broadcasts an "offset" carrier frequency, it takes a long time for fine tuning when channel selection is automatically performed, for example, by using an AFT circuit in accordance with the center frequency. For example, as shown in FIG. 2, if the broadcast "offset" carrier frequency is f.sub.1, but the center frequency is f.sub.0, the receiver PLL gradually drives the reference frequency from f.sub.0 to f.sub.1 over a time .DELTA.t, which is equal to the difference between t.sub.0 and t.sub.1, where t.sub.0 is the usual time for channel selection and t.sub.1 is the time at which channel selection occurs in this case.